The invention relates to protecting explosives, such as explosives used in downhole environments.
One operation that is performed in completing a well is the creation of perforations in a formation. This is typically done by lowering a perforating gun string to a desired depth in a wellbore and activating the gun string to fire shaped charges. The shaped charges when fired create perforating jets that form holes in surrounding easing as well as extend perforations into the surrounding formation.
Various types of perforating guns exist. One type of perforating gun includes capsule shaped charges that are mounted on a strip in various patterns. The capsule shaped charges are protected by individual containers or capsules from the harsh wellbore environment. Another type of perforating gun includes non-capsule shaped charges, which are loaded into a sealed carrier for protection. Such perforating guns are sometimes also referred to as hollow carrier guns. The non-capsule shaped charges of such hollow carrier guns may be mounted in a loading tube that is contained inside the carrier, with each shaped charge connected to a detonating cord. When activated, a detonation wave is initiated in the detonating cord to fire the shaped charges. In a hollow-carrier gun, charges shoot through the carrier into the surrounding casing formation.
The reliability of wellbore perforating guns depends on the mechanical properties and performance of many precise components and materials that are exposed to hostile conditions (e.g., high temperatures, mechanical shock and vibration, and so forth). Explosive components may also be degraded by water or vapor and other corrosive gases or liquids that are generated within the guns themselves. Typical explosive components in a perforating gun includes shaped charges and detonating cords. As shown in FIG. 1, a shaped charge 10 typically includes a main explosive charge 16 and a metallic liner 20, both contained in an outer case 12. A primer charge 14 coupled to the back of the main explosive charge 16 is ballistically connected to a detonating cord 24. A detonation wave traveling down the detonating cord 24 transfers energy to the primer charge 14, which in turn initiates the main explosive 16. Detonation of the main explosive 16 causes the liner 20 to collapse to form a perforating jet.
The following are examples of damage that may be caused to explosive components in a corrosive environment, which may contain water vapor and other gases. The outer jacket of the detonating cord may be damaged, which may increase the likelihood that the detonating cord may break resulting in the guns not firing. Damage to the outer jacket of a detonating cord may also be a safety hazard. The detonating cord may be accidentally pinched which may cause it to initiate.
The corrosive environment also desensitizes explosive materials in the detonating cords, shaped charges, or other components, which may cause a perforating gun to not fire. When a perforating gun string is lowered to a desired depth but for some reason cannot be activated, a mis-run has occurred. This requires that the perforating gun string be pulled out of the wellbore and replaced with a new gun string, which is time consuming and expensive. Also, retrieving a mis-fired gun from a wellbore may be a hazardous operation.
In addition, an explosive has a certain range of time and temperature in which the explosive is thermally stable. If the explosive is stretched beyond this range, the explosive starts to decompose, bum, or auto-detonate. The presence of water vapor acts as a catalyst that further accelerates the rate of decomposition of the explosive. Other products of decomposition may also act as catalysts in accelerating the decomposition.
A need thus exists for a method and apparatus to protect explosives in a corrosive environment and to reduce effects of explosive decomposition which may occur downhole or at the surface.
In general, according to one embodiment, an apparatus includes a housing, an explosive in the housing, and a material placed in the housing and in the proximity of the explosive to remove corrosive fluid to protect the explosive.
Other embodiments and features will become apparent from the following description, from the drawings, and from the claims.